QFlags tutorial: Difference between revisions

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[[Category:snippets]]
[[Category:Snippets::Misc]]
[[Category:Tutorial]]


[toc align_right="yes" depth="3"]
==Overview==


= Simple tutorial for safe-usage QFlags =
First of all we should write about the macro:
<code>Q_FLAGS</code>
''This macro registers one or several flags types to the meta-object system''


== Overview ==
==Example:==


First of all we should write about macro &lt;code&amp;gt;Q_FLAGS&amp;lt;/code&amp;gt;<br />''This macro registers one or several flags types to the meta-object system''
<code>class TestClass
{
public:
enum Option {
OptionA = 1, // 000001
OptionB = 2, // 000010
OptionC = 4, // 000100
OptionD = 8, // 001000
OptionE = 16, // 010000
OptionF = 32 // 100000
// … some more options with value which is a power of two
};
Q_DECLARE_FLAGS(Options, Option)
};


== Example: ==


<code>class TestClass<br /> {<br /> public:<br /> enum Option {<br /> OptionA = 1, // 000001<br /> OptionB = 2, // 000010<br /> OptionC = 4, // 000100<br /> OptionD = 8, // 001000<br /> OptionE = 16, // 010000<br /> OptionF = 32 // 100000<br /> // … some more options with value which is a power of two<br /> };<br /> Q_DECLARE_FLAGS(Options, Option)<br /> };
Q_DECLARE_OPERATORS_FOR_FLAGS(TestClass::Options)</code>


Q_DECLARE_OPERATORS_FOR_FLAGS(TestClass::Options)</code>
The '''Q_DECLARE_FLAGS()''' macro expands to '''typedef QFlags<Enum> Flags;''' In our case it expands to '''typedef QFlags<Option> Options;''' where '''Option''' - is an enum name, and '''Options''' - is name for set of flags.


The &lt;code&amp;gt;Q_DECLARE_FLAGS()&lt;/code&amp;gt; macro expands to <code>typedef QFlags&amp;lt;Enum&amp;gt; Flags;<code> In our case it expandes to &lt;code&amp;gt;typedef QFlags&amp;lt;Option&amp;gt; Options;&lt;/code&amp;gt; where '''Option''' - is an enum name, and '''Options''' - is name for set of flags.
The '''Q_DECLARE_OPERATORS_FOR_FLAGS()''' macro declares global '''operator|()''' functions for Flags, which is of type '''QFlags<T>'''.


The &lt;code&amp;gt;Q_DECLARE_OPERATORS_FOR_FLAGS()&lt;/code&amp;gt; macro declares global &lt;code&amp;gt;operator|()&lt;/code&amp;gt; functions for Flags, which is of type &lt;code&amp;gt;QFlags&amp;lt;T&amp;gt;&lt;/code&amp;gt;.
The '''Q_DECLARE_FLAGS()''' macro does not expose the flags to the meta-object system, so they cannot be used by Qt Script. To make the flags available for this purpose, the '''Q_FLAGS()''' macro must be used.


The &lt;code&amp;gt;Q_DECLARE_FLAGS()&lt;/code&amp;gt; macro does not expose the flags to the meta-object system, so they cannot be used by Qt Script. To make the flags available for these purpose, the &lt;code&amp;gt;Q_FLAGS()&lt;/code&amp;gt; macro must be used.
==Usage sample==


== Usage sample ==
<code>void test (TestClass::Options flag)
{
if (flag.testFlag(TestClass::OptionA))
qDebug() << "A";
if (flag.testFlag(TestClass::OptionB))
qDebug() << "B";
}


</code>void test (TestClass::Options flag)<br />{<br /> if (flag.testFlag(TestClass::OptionA))<br /> qDebug() &lt;&lt; &quot;A&amp;quot;;<br /> if (flag.testFlag(TestClass::OptionB))<br /> qDebug() &lt;&lt; &quot;B&amp;quot;;<br />}
int main()
{
test (TestClass::OptionA | TestClass::OptionB);
test (0x1); // error


int main()<br />{<br /> test (TestClass::OptionA | TestClass::OptionB);<br /> test (0x1); // error<br />}<code>
}</code>


&lt;code&amp;gt;testFlag(flag)&lt;/code&amp;gt; method checks if flag is set in &lt;code&amp;gt;QFlags&amp;lt;/code&amp;gt;.
The '''testFlag(flag)''' method checks if flag is set in '''QFlags'''.


=== Some example ===
===Some examples===


</code>TestClass::Options f1(TestClass::OptionA | TestClass::OptionB); // 000011<br />TestClass::Options f2(~f1); // 111100<br />TestClass::Options f3(Foo::OptionA | Foo::OptionC); // 000101<br />TestClass::Options f4(f1^f3); // 000110
<code>TestClass::Options f1(TestClass::OptionA | TestClass::OptionB); // 000011
TestClass::Options f2(~f1); // 111100
TestClass::Options f3(Foo::OptionA | Foo::OptionC); // 000101
TestClass::Options f4(f1^f3); // 000110
f4.setFlag(TestClass::OptionF).setFlag(TestClass::OptionB, false); //100100, introduced in Qt 5.7
</code>

Latest revision as of 20:52, 27 October 2020


Overview

First of all we should write about the macro: 

Q_FLAGS

This macro registers one or several flags types to the meta-object system

Example:

class TestClass
 {
 public:
 enum Option {
 OptionA = 1, // 000001
 OptionB = 2, // 000010
 OptionC = 4, // 000100
 OptionD = 8, // 001000
 OptionE = 16, // 010000
 OptionF = 32 // 100000
 // … some more options with value which is a power of two
 };
 Q_DECLARE_FLAGS(Options, Option)
 };


Q_DECLARE_OPERATORS_FOR_FLAGS(TestClass::Options)

The Q_DECLARE_FLAGS() macro expands to typedef QFlags<Enum> Flags; In our case it expands to typedef QFlags<Option> Options; where Option - is an enum name, and Options - is name for set of flags.

The Q_DECLARE_OPERATORS_FOR_FLAGS() macro declares global operator|() functions for Flags, which is of type QFlags<T>.

The Q_DECLARE_FLAGS() macro does not expose the flags to the meta-object system, so they cannot be used by Qt Script. To make the flags available for this purpose, the Q_FLAGS() macro must be used.

Usage sample

void test (TestClass::Options flag)
{
 if (flag.testFlag(TestClass::OptionA))
 qDebug() << "A";
 if (flag.testFlag(TestClass::OptionB))
 qDebug() << "B";
}

int main()
{
 test (TestClass::OptionA | TestClass::OptionB);
 test (0x1); // error

}

The testFlag(flag) method checks if flag is set in QFlags.

Some examples

TestClass::Options f1(TestClass::OptionA | TestClass::OptionB); // 000011
TestClass::Options f2(~f1); // 111100
TestClass::Options f3(Foo::OptionA | Foo::OptionC); // 000101
TestClass::Options f4(f1^f3); // 000110
f4.setFlag(TestClass::OptionF).setFlag(TestClass::OptionB, false); //100100, introduced in Qt 5.7
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